A base for a neighborhood system of a point 
is a collection 
of open sets such that 
belongs to every member of 
, and any open set containing 
also contains a member of 
as a subset.
Neighborhood System Base
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Cite this as:
Weisstein, Eric W. "Neighborhood System Base." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/NeighborhoodSystemBase.html